The calendar we use today, dates back to 1582, to Pope Gregory XIII, and is
often referred to as the Gregorian Calendar. When Pope Gregory XIII introduced
this new revised calendar he was required to add ten days from the then current
date of October 5th, 1582, to bring it into alignment with his new calendar.
The reason for this was because it had been determined that the previous calendar,
the Julian Calendar, overestimated the true calendar year by a factor of 10
minutes and 48 seconds every year. But it wasn't to be until 1752 that Great
Britain and the American colonies accepted the Gregorian calendar.

The concept of a calendar

The whole concept of a calendar is clearly based upon astronomical events like
the rotation of the earth, the cycle of the moon and the time taken for the
earth to journey around the sun. The understanding of these factors is crucial
to creating an accurate calendar. The time it takes for the earth to complete
one full revolution of the sun, as measured from one equinox to the next, is
approximately 365.2 days (although it varies) and is referred to as a 'tropical'
year. As any astronomer will know, this is not quite an accurate description
of a year because of a factor called 'precession' which accounts for the 'wobble'
in the earth's rotational axis.

A month used to be based on the time it takes for the moon to complete a full
revolution of the earth, called the 'synodic' month. This period also varies
depending on the distance of the moon from the earth and is currently about
29.5 days. In the Gregorian calendar, this relationship has finally been broken
due of the lack of relationship between the synodic month and the tropical year.
There is a relationship though, as a Greek astronomer discovered in the 5th
century. Almost exactly 235 synodic months equals nineteen tropical years so
the cycle of the moon would repeat if it were not for the leap year. Other factors
can change these average periods from year to year and month to month like the
gravitational pull of the planets and the sun, making the construction of an
astronomically accurate calendar, practically impossible.

Effectively, the Gregorian calendar is a 'solar' calendar designed to stay
in synchronization with the tropical year. The reason we need the leap year
every forth Gregorian year is to try to increase the average length of the Gregorian
year.

The Islamic calendar is a lunar calendar, paying no regard to the tropical
year which is why set religious holidays like Ramadan, drift across the Gregorian
calendar and the tropical year from year to year.

The third type of calendar is the lunisolar calendar. This type of calendar
has a 'leap' month every few years to bring the whole calendar back in phase
with the tropical year. Examples of this type of calendar would be the Chinese
calendar and the Hebrew calendar.

From the above it can be seen that calendars were created to serve man and
not the other way around. The accuracy of a calendar to astronomical events,
which are not exactly know, is only one factor to be considered in creating
a suitable calendar for social use.

Origin of our calendar

Before the Gregorian calendar was the Julian calendar. The Julian calendar
was put in place by Julius Caesar in 40 BC where it remained in use until as
late as 1914 in Russia. It was surprisingly accurate for its early age stating
a year to be 365 1/4 days. Despite this accuracy it did slowly drift off track
over the ensuing centuries.

In the Middle Ages, any form of study into the measurement of time was considered
to be 'prying into God's affairs' and therefore frown upon. Because monks were
largely the scholars of the day the calendar got very little attention past
working out the dates for religious events. It took until 1582 for Pope Gregory
XIII too finally reform the Julian calendar and by then it was a full ten days
out of alignment. In fact, considering how aggressively the Catholic Church
condemned Galileo for his views on the workings of the solar system, it's surprising
that they played such an active roll in the development of the new calendar.
History tells us that Pope Gregory XIII was attached by many Christians of the
day over his new calendar, calling him the 'Roman Antichrist' and suggesting
that he was trying to lead them to worship on the wrong days. For this reason
the new Gregorian calendar was not fully adopted across Europe until way into
the seventeen hundreds.

The Gregorian calendar was devised by Aloysius Lilius, a doctor from Naples,
Italy. In his new calendar the number of days in a year is approximated to 365
97/400 days. In other words there would be 97 leap years every 400 years to
keep the calendar on track with the tropical year. Thus it was to take about
3,300 years before the Gregorian calendar would be one day out of step with
the tropical year.

Using the original Gregorian calendar, New Year's Day was designated to be
the day that coincided with the autumn equinox. Every forth year contained an
extra day to try to ensure the consistency of New Year's Day on this autumn
equinox every year. This has long since been dropped due to the difficulties
of predicting the autumn equinox.

The calendar that we use today is the result of a long human chain of scientific
process and invention spreading back through time from our clocks and watches
to the dawn of mankind. Originally a human form must have looked up at the night
sky and stared at the moon and devised a way to tell the passage of time. Through
the refinements of ancient Egypt, then the Roman Empire, and on up through the
Middle Ages with the spread of knowledge from the east and from the middle east,
the west was to develop the calendar with it's demand for social structure.

America, to this day, does not stipulate or legally designate any official
calendar to be used in this country. The only reason we use the Gregorian calendar
today is because of an Act of Parliament in the United Kingdom in 1751, which
mandated the use of the Gregorian calendar in England and all of its colonies.

When you look down at your Bulova watch, spare a thought for the evolution
of time and the struggles to develop a calendar to bring you that small aperture
on the face displaying the date.